1. Field of the Invention
The present invention generally relates to the editing of a magnetic recording of a video signal, and more particularly to an electronic editing method and apparatus for video tape recorders wherein a video signal is recorded successively onto a prerecorded video signal existing on a magnetic tape (hereinafter referred to as "prerecorded video signal" or "old signal" in the specification) in such a manner that, when played back, no picture disturbance is produced.
2. Description of the Prior Art
Electronic editing systems are utilized in video tape recorders for domestic use as well as broadcasting use. In the past, there have been two kinds of such electronic editing systems, one being an "insert editing system" in which a new viedo signal was inserted in the old video recording and the other being an "assembly editing system" in which a new video signal was recorded successively to the prerecorded video signal with continuity. The former editing system was principally utilized with television broadcasting techniques and the latter editing system was utilized both in such broadcasting use and in domestic use. In both uses, however, recording of a new video signal had to be commenced under the strict condition that no difference of phase existed or precise synchronism had to be ensured between the old prerecorded video signal and the new video signal. If any such difference existed therebetween, synchronism of the signals was disturbed at an edited transition of a tape with the result that a picture would also be disturbed in the played back image.
At the initial stage of such an editing technique, an editing of a magnetic recording was mechanically accomplished by a method in which the tape was cut at a proper point of the recorded video signal and tape segments were spliced together to provide an edited tape. Subsequent to such mechanical editing, an electronic editing technique was proposed whereby in order to record a new video signal successively to the old video signal prerecorded on a magnetic tape, the control signals also recorded on the magnetic tape coincidentally with the old video signal were picked up in a playback mode and compared in phase with vertical synchronizing signals contained in the new video signal. Any phase difference thus obtained served to control capstan servos so that the running speed of the magnetic tape was so adjusted as to bring the control signals in phase with the synchronizing signals and, upon coincidence of the phases therebetween, the operational mode of the video tape recorder was switched from a playback mode to a recording mode to start the actual recording of the new video signal. In this way, electronic editing was successfully accomplished. When the magnetic tape thus edited was played back, no appreciable disturbance of synchronism was recognized at the edited transition of the tape because of complete coincidence of phases between the old and the new video signals. Thus, the desired playback picture was obtained.
In the electronic editing technique above outlined, it was necessary to playback the magnetic tape recording containing the old video signal to pick up the control signals. In the "insert" editing operation wherein the new video signal was inserted in the old video signal, the control signals were already recorded with the old video signal on the section of the magnetic tape on which the new video signal was to be recorded, and therefore no rewinding of the magnetic tape was needed to pick up the control signals so that only playback was therefore sufficient. In the "assembly" editing operation wherein a new video signal was recorded successively to the end of an old video signal prerecorded on the tape, no video signal was contained or only the video signal recorded prior to the old video signal was contained in the section of the tape on which the new video signal was to be recorded, and therefore no usable control signals for electronic editing could be picked up.
In such an assembly editing operation, there has been no proposal as to how to realize the editing operation, except in some technical publications suggesting that a part of the magnetic tape containing the old video signal was first rewound and then played back to pick up the control signals. These publications, however, did not disclose when or how rewinding of the magnetic tape was commenced, much less how such rewinding period or the rewound length of the tape was determined.